Lightweight design of structural vehicle components has been more important. For example, hybrid structures of steel and aluminium or composites yield a weight advantage compared to conventional steel structures. However, these secondary materials such as aluminium or composites cannot be joined to steel using resistance spot welding.
In a certain example, U.S. Pat. No. 5,100,494 discloses a process for temporarily joining and then separating two pieces of material. The process comprises steps of furnishing two pieces of material having conformably shaped facing surfaces, bonding the two pieces of material together at a joint, and passing a sufficient electrical current through the resistance element to melt the thermoplastic adhesive, thereby separating the two pieces. The joint includes a layer of thermoplastic adhesive, an electrical resistance element between the two pieces, and means for separating and spacing the electrical resistance element apart from each of the two pieces of material that are bonded together, and the means for separating and spacing apart includes a plurality of non-conductive spacers placed between the electrical resistance element and the pieces of material on each side of the electrical resistance element.
In addition, U.S. Pat. No. 6,146,488 discloses a weld bonding method for bonding metal surfaces together with adhesion both by an adhesive and by resistance spot welding through the adhesive. The process comprises steps of preparing the adhesive by admixing a thermosetting epoxy resin, a latent curing agent and 1 to 15 vol % of one or more conductive additives selected from the group consisting of metal, metal oxide, metal carbide, metal nitride, metal boride and metal silicide; applying the adhesive to at least one of the metal surfaces to be bonded, then spreading the applied adhesive; placing the metal surfaces together with the spread adhesive there between; spot welding the metal surfaces together; and heating the metal surfaces to cure the adhesive whereby the additive improves conductivity to enhance the strength of the spot welds. The additive is in the form of a powder having a particle size of 10 μm or less, or in the form of fragments or flakes having a thickness of at least 0.5 μm and a particle size of 30 μm or less,
The usage of an adhesive bonding joining agent can provide an efficient hybrid joint performance. The adhesive needs to be partly cured to allow parts handling. Therefore, a new process is needed to allow pre-curing of adhesive before the main curing process.